Key Difference – Hardness vs Toughness
Hardness and Toughness, though the two words are synonyms according to some standard dictionaries, there is a key difference between them in the study of material science. In general, a solid material, depending on the force applied on it, exhibits three types of changes; elastic changes, plastic changes, and fraction. For a solid material, hardness and toughness values depend on the elasticity, plasticity and fraction. The key difference between hardness and toughness is that these two properties of materials have an inverse relationship. For a particular solid material; as hardness increases, toughness decreases. Hardness is a measure of a material’s resistance to permanent deformation. Toughness is a measure of how much deformation a solid material can undergo before fracturing. Therefore, it can be said that hardness and toughness have an inverse relationship. For a particular solid; hardness increases as toughness decreases.
What is Hardness?
Hardness is a measure of a material’s resistance to plastic deformation. This property is closely related to the strength; the ability of a material to resist scratching, abrasion, indentation, or penetration. The common hard materials are; ceramics, concrete, and some metals.
What is Toughness?
Toughness is a measure of how much deformation, a material can undergo before fracture. In other words, it is the ability to withstand both plastic and elastic deformations. This material quality is very important for structural and machine parts to endure shock and vibration. Some examples of tough material are, manganese, wrought iron and mild steel. For example, if we apply a sudden load to a mild steel piece and a glass, the steel material will absorb more energy than the glass before it fractures. Therefore, mild steel material is said to be much tougher than glass material.
What is the difference between Hardness and Toughness?
Definition of Hardness and Toughness
Hardness: Hardness is a parameter which measures how resistant a solid material is to permanent shape changes when a compressive force is applied. Hard materials usually have strong intermolecular forces. Therefore, they can withstand external forces without changing their shape permanently.
There are several measurements of hardness, to understand the complex behavior of solid matters under a force. They are scratch hardness, indentation hardness, and rebound hardness.
Toughness: In material science and metallurgy, toughness is described as the ability of a material to absorb energy to deform plastically without fracturing. It is also said to be the resistance to deforming plastically, before fracturing when stressed. Sometimes, it is defined as the energy per unit volume that a material can absorb without rupturing.
SI units = joule per cubic metre (J m−3)
Properties and Examples of Hardness and Toughness
Hardness: A hard material can scratch a soft material. Hardness depends on other material properties such as ductility, elastic stiffness, plasticity, strain, strength, toughness and viscosity. Diamond is the hardest natural material on the earth. The other examples of hard materials are ceramics, concrete, and some metals.
Toughness: Tough material can absorb large amounts of energy without fracturing; therefore tough materials require a balance of strength and ductility. Brittle materials have a lower value for toughness. Manganese, wrought iron, and mild steel materials are considered as tough materials.
Hardness and Toughness Tests
Hardness: Three main types of hardness values are measured in three different ways to measure scratch hardness, indentation hardness and rebound hardness.
|Type||Scales of measurements / instruments|
|Scratch hardness||Sclerometer – Mohs scale and pocket hardness tester|
|Indentation hardness||Rockwell, Vickers, Shore, and Brinell scale|
Toughness: The simple way of measuring toughness value of a solid material is just measuring the energy needed to break the material. This requires a small sample of the material, a fixed size with a machine’s notch. This method cannot be used to all materials, but useful to rank materials that are used in products which undergo pressure. (generally metals).Image Courtesy: “Diamonds” by Swamibu (CC BY 2.0) via Commons “Mangan 1-crop” by Tomihahndorf – Mangan 1.jpg.(CC BY-SA 3.0) via Commons “Stress-strain1″ by Moondoggy – . (CC BY-SA 3.0) via Commons